The present invention relates to a vacuum brake booster for a motor vehicle brake system, comprising a displaceable force input member which is coupled or is capable of being coupled with a brake pedal, a chamber arrangement being arranged in a booster housing, having at least one working chamber and at least one vacuum chamber which are separated from one another via at least one movable wall, a control valve assembly which is actuatable by the force input member, and a force output member for transferring an actuating force to a downstream brake system, wherein the at least one working chamber is optionally fluidically connectable to a vacuum power source or the atmosphere via the control valve assembly, and wherein the control valve assembly is connected to the at least one movable wall for joint movement, wherein the control valve assembly comprises: a control valve housing in which an actuating piston is arranged displaceably, the actuating piston being coupled to the force input member, and a first valve seat and a second valve seat, wherein the control valve assembly is biased into a normal position in which at least the first valve seat is closed.
Such vacuum brake boosters are well known from the prior art. By fluidically connecting the at least one working chamber either to a vacuum power source, for example the intake manifold of a combustion engine, or to the surrounding atmosphere by means of the control valve assembly, a pressure difference can be built up at the at least one movable wall as required. This pressure difference can boost the brake force exerted on the brake pedal by the driver and thus assist with the braking process. In particular, the fluidical connection between the at least one working chamber and the surrounding atmosphere can be established by opening a valve seat, which is here referred to as the first valve seat. Correspondingly, the at least one working chamber can be connected to the vacuum power source or separated from the vacuum power source via the valve seat referred to as the second valve seat.
It has been shown that, when the first valve seat is opened, instabilities in the air stream that develops frequently occur in the air gap that forms. These instabilities can manifest themselves as vibrations at the brake pedal that are noticeable to the driver and/or as noise in the footwell of the vehicle. The mentioned instabilities can develop not only at the start of braking but also when braking is increased in its intensity from a holding phase. In other words, these instabilities occur whenever the control valve assembly is displaced from an equilibrium position, which is generally referred to here as the normal position and in which the first valve seat and the second valve seat are closed, in order to open the first valve seat.
Since the described instabilities occur predominantly at high flow rates in the region of the control valve assembly, attempts are made in the prior art to restrict the supply of air from the region of the surrounding atmosphere in the affected portion of the control valve assembly or of the brake booster.
Such a vacuum brake booster is known, for example, from document WO 00/18627 A1. This document describes a vacuum brake booster having a control valve assembly which comprises a flexible air filter. The air filter is arranged in or on a displaceable actuating piston of the control valve assembly. In conjunction with a baffle plate and an associated valve seat it forms a changeable inflow cross-section for ambient air flowing through the valve assembly. The baffle plate has a protruding stop edge which plunges into the flexible air filter to a predetermined degree when the valve seat is closed. If the valve seat is opened by displacement of the actuating piston and the air filter thus moves away from the baffle plate, or the stop edge thereof, the degree by which the stop edge plunges into the air filter is reduced. As long as the stop edge of the baffle plate and the air filter act together, that is to say as long as the stop edge is not lifted from the air filter completely, the ambient air flowing in from outside is conducted by the baffle plate through the filter and is thereby restricted. Owing to the arrangement of the baffle plate, the air filter and the valve seat, the inflowing air is guided substantially in an S-shape through the air filter.
Further prior art is known from document EP 1 323 606 B1. This document describes a vacuum brake booster having a control valve assembly which comprises a resilient flow control valve for the known valve seats. The flow control valve is preferably made of a rubber-elastic material and has a radial opening. The flow control valve is connected in an interlocking manner to a valve element and is fixed relative thereto in its position relative to the valve element. The flow control valve is sectionally in contact with a displaceable actuating piston of the control valve assembly. If the actuating piston is moved away from the flow control valve in the actuating direction, the first valve seat opens in order to connect a working chamber with the surrounding atmosphere. Since the flow control valve is resiliently deformed for sectional contact with the actuating piston, the flow control valve initially remains in contact with the actuating piston when the above-mentioned first valve seat is opened. Ambient air thereby initially flows via the opening of the flow control valve through the open first valve seat into the working chamber. During this process, the flow control valve is not displaced relative to a housing of the control valve assembly and relative to the valve element.
The solutions from the prior art use components which, because of their material properties, are susceptible to contamination and/or wear. As the operating time increases, this has negative effects on the functioning of the brake booster in question. For example, an air filter can become at least partially blocked by impurities in the ambient air and restrict the flowing air stream too greatly. The material that is used for the air filter can also become porous as the operating time increases. The result in both cases is that the intended function of the control valve assembly can no longer be ensured sufficiently. The same is true for a rubber-elastic flow restricting valve whose resilient material properties become worse over time. This can lead to a loss of the resilient preloading of the flow restricting valve relative to the actuating piston, which likewise has a negative effect on the functioning of the control valve assembly, or on the flow restricting behavior.